This disclosure relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an example described below, more particularly provides resolution enhancement for distributed optical measurements.
Distributed temperature sensing (DTS) is a technology that can be used to measure temperature distribution along an optical waveguide (such as an optical fiber, optical ribbon, etc.). A pulsed laser source is used to send a pulse of light through the optical waveguide, and properties of returning light are recorded. The returning light, “backscatter,” comprises absorption and retransmission of light energy.
The backscattered light includes different spectral components, e.g., Rayleigh, Brillouin, and Raman bands. The Raman band can be used to obtain temperature information along the fiber.
The Raman backscatter has two components, Stokes and Anti-Stokes, the former being weakly dependent on temperature and the latter being greatly influenced by temperature. The relative intensity between the Stokes and Anti-Stokes components is a function of temperature at which the backscattering occurs.
Since the speed of light in glass is known, it is possible to determine, by tracking the arrival time of the reflected and backscattered light, the precise location where the backscattered light originated. A DTS trace or profile is a set of temperature measurements or sample points, equally spaced along the waveguide length.
Every sample point represents the average temperature along a length section called a “sampling interval,” and during a period of time called “acquisition time.” The sampling interval is usually about 1 meter and the measurement time can range from a few seconds to several minutes, and even hours.
Unfortunately, in high flow rate circumstances (such as during fracturing, injection, other stimulation operations, gas production, etc.), fluid can travel a substantial distance over a short period of time. Thus, it will be appreciated that conventional methods of distributed temperature measurement for tracking fluid movement along a wellbore could be improved.
Such improvements would be useful in situations where temperature profiles change significantly in a short timespan, for example, where it is desired to track a thermal tracer which is displacing at a relatively high velocity. The long measurement times used in conventional DTS systems cannot provide sufficiently high resolution to track high velocity events.
Other optical measurement techniques, such as distributed acoustic sensing (DAS), distributed vibration sensing (DVS), etc., could also benefit from improvements in measurement resolution.